Multispectral photon detection (covering the visible and infrared) allows target identification with enhanced information content. For example, in military applications, multispectral-photodetector arrays can be used to identify real missile heads from fake ones and can help direct the antimissile head toward an actual target. The principle of multispectral detection has been widely used in astronomic observations. One available photodetection mechanism is electron-hole generation in a reverse-biased PIN diode
For PIN diode-based photodetectors, the longest wavelength that can be detected is dictated by the bandgap of the intrinsic semiconductor used for the intrinsic layer. Photons with energy higher than the bandgap energy will be detected, as long as the intrinsic layer is sufficiently thick to absorb the photons. The photodetectors must provide high efficiency and high-speed operation in order to use them for high-speed object detection. To integrate photodetectors on a CMOS chip, where high speed and sophisticated digital signal processing circuitry can be made, Group IV-based detectors may be desirable. The current approaches of making photodetectors on a Si chip generally employ thick Si PIN diodes that are vertically buried deep in the Si substrate. For these PIN diode photodetectors, efficiency can be kept high as long as the intrinsic layer is thick enough. However, the speed of these PIN diodes is rather low, because of the long tail of collection of photon current (due to the low drift velocity of holes).
Ge is a promising material for light detection in optical communication for at least two reasons. First, because of its direct bandgap at 0.8 eV and indirect bandgap of 0.66 eV, it is highly absorbing over a wide range. Second, because of its compatibility with existing Si technology, it offers the potential for high-quality CMOS compatible integrated photoreceivers. Unfortunately, due to the large lattice mismatch between Si and Ge, progress toward the formation of Ge-based high-speed photodetectors on Si has been very limited.
Some work has been done to fabricate photodetectors using growth of Ge quantum dots on Si surfaces to make many alternating layers of Si film and Ge quantum dots. Although such photodetectors provide some increase in absorption, they show only a slight improvement over Si PIN diodes.